1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and an electrophotographic image forming method, and more particularly to an electrophotographic image forming apparatus and an electrophotographic image forming method, which produce images using an intermediate transfer medium.
2. Discussion of the Background
Recently, electrophotographic image forming apparatuses (such as copiers, printers and facsimile machines) capable of forming multi color images or full color images have been commercialized. These color image forming apparatuses typically include the following processes:    (1) cyan (C), magenta (M), yellow (Y) and black (Bk) toner images are formed on an image bearing member or respective image bearing members such as photoreceptors (toner image forming process);    (2) the color toner images are transferred one by one on an intermediate transfer medium to form a full color toner image on the intermediate transfer medium (primary transfer process);    (3) the full color image is then transferred to a receiving material such as papers (secondary transfer process); and    (4) the full color image is fixed to the receiving material (fixing process); and    (5) the surfaces of the image bearing member and the intermediate transfer medium are cleaned with cleaners (cleaning process)
Recently, tandem-type full color image forming apparatuses, which use four image bearing members for forming C, M, Y and Bk color images, respectively, and an intermediate transfer medium, are widely used because of having image productivity.
In addition, in order to produce high quality images and reduce consumption of energy used for producing toner, polymerization toners, in which toner particles are produced by a polymerization method, are widely used for such color image forming apparatuses.
When images are produced using a toner, a problem in that a toner image on an image bearing member is non-electrostatically (i.e., physically) adhered thereto if a relatively high pressure is applied to the toner image in the primary transfer process occurs, and the toner image remains on the image bearing member without being transferred by an electrostatic force formed by applying a transfer bias. Particularly, since a relatively high pressure is applied to center portions of character images and line images compared to the edge portions thereof, an image omission problem in that the center portions character or line images are omitted is easily caused.
In order to solve this image omission problem on the toner side, it is preferable to reduce the adhesiveness of the toner to the surfaces of the photoreceptor and the intermediate transfer medium used and/or to reduce adhesiveness among toner particles. Specifically, it is preferable to use one or more of the following techniques.    (1) A round toner is used;    (2) A toner including a hard resin is used;    (3) A toner including a large amount of external additive is used; and    (4) A toner including a large external additive is used.
However, when such toners are used, other problems such that the toners have poor cleanability and fixability tend to be caused. In other words, it is difficult to provide a toner which does not cause the image omission problem and has a good combination of cleanability and fixability. Specifically, the content of an external additive in a toner has to be decreased to improve the fixability of the toner, and the content has to be increased to improve the transferability of the toner.
In order to solve the image omission problem on the image forming apparatus side, the following techniques are used.    (1) The pressure (i.e., transfer pressure) applied to a toner image is decreased in the primary transfer process;    (2) The linear velocity of the image bearing member is controlled so as to be differentiated from that of the intermediate transfer medium;    (3) A lubricant is applied to the surface of the image bearing member to decrease the static friction coefficient thereof while no lubricant is applied to the intermediate transfer medium;    (4) An intermediate transfer medium having an adhesive layer thereon is used to improve the toner image transfer efficiency of from an image bearing member to the intermediate transfer medium and the toner image transfer efficiencies in the primary and secondary transfer processes (published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 59-50475);    (5) A friction coefficient decreasing material is included in an intermediate transfer medium to decrease the surface thereof, which results in prevention of occurrence of the image omission problem (JP-A 09-34276);    (6) An elastic layer is formed in an intermediate transfer medium while a lubricant is applied to the surface of the intermediate transfer medium (JP-A 2003-29550); and    (7) A protective layer including a material having good lubricity is formed on the surface of the intermediate transfer medium to improve the toner image transfer efficiencies in the primary and secondary transfer processes (JP-A 02-213881).
When the technique (1) in that the transfer pressure is decreased is used, a transfer nip cannot be stably formed and thereby a uniform transfer bias cannot be applied to the image bearing member and intermediate transfer medium, resulting in occurrence of an undesired image transfer problem in that a toner image cannot be transferred to a predetermined position. Therefore, the transfer pressure cannot be largely decreased, and thereby the image omission problem cannot be perfectly solved by this technique.
When the technique (2) in that the linear velocity of the image bearing member is differentiated from that of the intermediate transfer medium is used, an undesired image transfer problem in that a toner image cannot be transferred to a predetermined position and/or the transferred image is rubbed, resulting in formation of a damaged image occurs. As a result of the present inventor's study, the linear velocity can be differentiated only by about 1%. In this case, the image omission problem cannot be perfectly solved.
When the friction coefficient of the surface of the image bearing member is high, toner particles remaining on the image bearing member cannot be well removed therefrom with a cleaning blade. This cleaning problem is dominantly caused when the toner is a polymerized toner which typically has a high circularity.
In attempting to solve the problems, various techniques have been proposed. However, there is no technique by which occurrence of the problems can be stably prevented for a long period of time.
Because of these reasons, a need exists for an image forming apparatus which can produce high quality images without causing image omission problem in that toner images on an intermediate transfer medium are not well transferred to a receiving material and the cleaning problem in that toner particles remaining on the intermediate transfer medium are not well removed therefrom with a cleaning blade.